Technical Field of Invention
The invention disclosed and taught herein relates generally to a system and method for use in floating offshore environments including drilling rigs. The embodiments described below related generally to the design layout of equipment on an offshore Well Intervention Semisubmersible (WIS) platform specifically for the transport, deployment, and retrieval of well intervention subsea equipment.
Background of the Invention
A typical layout of an exploratory drilling rig is to place a pyramid type drilling derrick at or near the center of the upper deck with an “elevated drill floor.” These type of derricks are usually built of truss structures with the drilling equipment installed within its footprint boundary. A drilling riser would be installed through the rotary table on the drill floor and connected to the subsea well. It is essentially a conduit for running the drill string to the well below and allowing the return of mud flow through the riser annulus to the surface. At the bottom of the drilling riser, there is a safety device called the subsea BOP Stack (Blow Out Preventer) latched on the wellhead and the LMRP (Lower Marine Riser Package). The BOP stack is designed to have the mechanism to shear off the drill string and shut in the well for well control purpose. Whereas the LMRP provides the quick disconnect mechanism of the riser on the top of the BOP in the event the drilling vessel is required to move away from the well that is out of control. The typical size of a deep water drilling riser joint has about a 21 inch outer diameter, about 75 feet in length, and with over about 50 inch of diameter for the buoyancy material attached. For deep water application, the combined BOP stack and LMRP can reach over 60 feet tall and the combined weight over 400 metric tonnes depending on the number of rams being configured on the BOP. Accordingly, the equipment for assembling, handling, transporting, and positioning the massive BOP stack and the LMRP underneath the drill floor becomes the center issue in the drilling system design layout. In order to provide sufficient head room for this operation, an elevated drill floor is normally required in the layout. Such an elevated drill floor is supported by the derrick substructures on the upper deck.
The design of a Well Intervention Semisubmersible, however, is not intended for drilling of a new well to the reservoir formation. Instead, its primary function is to provide down hole work-over service of a well that has been produced for a period of time. A typical well intervention operation can be performed by means of different methods including slick line, wire line, and coiled tubing deployment through the riser. The corresponding well intervention riser has a smaller diameter in comparison with a drilling riser, usually in the range of 7 to about 8.5 inch outer diameter. Unlike the drilling of an exploratory or development well, the physical characteristics of the well and the composition of the well stream are usually known prior to the well intervention operation. The use of a full size drilling BOP stack and LMRP is considered as overkill for well intervention. Therefore a need exists to provide for a vessel with a non-elevated derrick.